The present invention relates to apparatus for preventing light fogging of unexposed photographic film in its dispenser, particularly as encountered in archival microfiche film systems employing an intermediate film strip to carry a transferable image.
There has been developed by the applicants and co-workers an archival hard copy microfiche recording system which reflects light off of a document placed on a document-receiving platform and projects an image of it in greatly reduced form onto an initially transparent intermediate dry silver film strip carried by a film head unit. The film strip is dispensed from a cassette mounted on the film head, and is fed by film advancing means to an imaging region where a selected area of the film strip to be exposed can receive the projected light image. A pressure element presses the selected area of the film strip against a planar backing aperture defining a projection plane and an imaging area, and the film is then exposed to the light image. The film head is next moved along guides from the imaging station to a developing station, where a hot shoe pressing against the exposed area of the film causes development of the image to form an opaque image in the exposed areas thereof. The film head is then moved to an image transfer station, where the image on the dry silver film is to be replicated on a positive initially opaque microfiche film card of the photo-developing type (See U.S. Pat. No. 4,137,078 issued June 30, 1979 to Izu and Ovshinsky). The microfiche card is held indexed on a movable carriage so that a predesignated frame of the card receives the image on the intermediate film. A pressure element presses the film card and the intermediate film strip together between the output prism of a flash lamp housing and a backing plate. Image transfer is accomplished by triggering the flash lamp to direct light through the intermediate film, immediately rendering transparent the previously opaque areas struck by the high intensity flash lamp light on the selected frame of the microfiche film card. To replicate a new image the film head is returned to the imaging station, where the cycle is repeated.
The microfiche film cards are of the threshold photodeveloping type, and require a relatively high radiant flux for complete exposure compared to the dry-silver intermediate film used. The transfer of the intermediate film image to the microfiche film is done by contact printing, using a high intensity flash lamp. A fraction of this illumination becomes trapped in the intermediate film, which is quite clear in unexposed regions, and proceeds to propagate laterally internally along the plane of the film by total internal reflection (light-piping), thereby making its way into unexposed film in the payout region of the film dispenser. A significant length of intermediate film thus becomes light struck in the film dispenser. This necessitates a long film advance between successive frames, and is thus wasteful of film.
This phenomenon is not significant during formation of the initial image because the requisite illumination level is much lower than for the transfer operation, owing to the relatively high speed of the intermediate film compared to the microfiche film. During the transfer operation the intermediate film is subjected to an illumination level several orders of magnitude higher than necessary for initial image formation, and the propagation of a small fraction of this by light-piping is sufficient to give rise to significant fogging effects.
Additionally, the intermediate film is similarly fogged by prolonged exposure of the cassette to the ambient light of the system. It is an object of this invention to protect the microfiche record against the effects of fogging from both sources.